CSC 453

Operating Systems

This class

Who am I?

https://pschmitt.net
Academics
- PhD at UCSB
- Researcher (summers) at ICSI
- Postdoc then research faculty at Princeton
- Some other stops… USC, Stanford, UH
Privacy startup co-founder & CEO, measurement startup COO
Main focuses
- Integrating privacy and security into systems that we all use
- ML for network management
- Connectivity / privacy in challenging environments

This class

Too much to cover in one term: survey of important topics
Course Canvas: https://canvas.calpoly.edu/courses/168889
Syllabus: https://schmittpaul.github.io/CSC453W26/syllabus.pdf
Waitlists…
What OSes do you all run?

Who are you?

Name / Major / Year
Where do you identify as your hometown?

How to succeed in this class

Turn things in on time
Attend class
- … on time. Lectures will start at 10 after the hour
Ask questions
- I’m very flexible about how much we cover this semester
- I would rather teach less and have everyone understand it
- Our back-and-forth during class is the one of the few indicators I have of how much you are absorbing
Talk to me if you are struggling

What are we doing here?

How many of you have participated in OS development?
How many of you regularly program in languages that use operating system abstractions directly?
And C is a decreasingly popular language!
So why study operating systems? Why is this class even offered? Why is it required?

Why take this class

You are required to do so in order to graduate
Reality: this is how computers really work, and as a computer scientist or engineer you should know how computers really work
Ubiquity: operating systems are everywhere and you are likely to eventually encounter them or their limitations
Beauty: operating systems are examples of mature solutions to difficult design and engineering problems. Studying them will improve your ability to design and implement abstractions

Course progression

Introduction to operating system abstractions and structures
Abstracting and multiplexing:
- CPU: interrupts, context, threads, processes, processor scheduling, thread synchronization, deadlocks
- Memory: memory layout, address translation, paging and segmentation, address spaces, translation caching, page fault handling, page eviction, swapping
- File systems and storage: disk scheduling, on-disk layout, files, buffer cache, crash and recovery
- Security (time permitting)

Background

Questions to consider

What is an operating system?
What do they do?
How did OSes evolve?
How do the requirements differ for different OSes?

What is an operating system?

Operating System:
- A computer program that
- multiplexes hardware resources and
- implements useful abstractions.
The OS is just another computer program. Has the highest privilege. The core of the OS consists of the kernel
Multiplexing allows multiple people or programs to use the same set of hardware resources - processors, memory, disks, network connection - safely and efficiently.
Abstractions simplify the usage of hardware resources by organizing information or implementing new capabilities.

OS history - how did we get here?

Started out as libraries to provide common functionality across programs
- See any issues with this approach?
Later, evolved from procedure call to system call
- What’s the difference?
Evolved from running a single program to multiple processes concurrently
- New issues to solve?

Types of operating systems

Desktop
Time share/Mainframe
Mobile
Web
Real-time
Embedded
Virtual Machines

Tl;dr

OSes continue to evolve as environments (i.e., constraints) change

What isn’t clear?

Comments? Thoughts?

OS abstractions

Questions to consider

What are the main abstractions OSes provide?
What are the abstraction challenges?

Abstractions

Abstractions simplify application design by:
- hiding undesirable properties,
- adding new capabilities, and
- organizing information
Abstractions provide an interface to application programmers that separates policy—what the interface commits to accomplishing—from mechanism—how the interface is implemented.

What are the abstractions?

CPUs
- Processes, threads
Memory
- Address space
Disk
- Files

Example OS abstraction: file systems

What undesirable properties do file systems hide?
- Disks are slow!
- Chunks of storage are actually distributed all over the disk
- Disk storage may fail!
What new capabilities do files add?
- Growth and shrinking
- Organization into directories, searchability
What information do files help organize?
- Ownership and permissions
- Access time, modification time, type, etc.

Abstraction tradeoffs - discussion

Identify undesirable properties hidden by, new capabilities added, and info organization provided with these abstractions:
- Process / threads
- Address space

Processes/Threads

Hiding undesirable properties:
- Example: The process abstraction hides the complexity of CPU scheduling and context switching. Applications don’t need to manage the low-level details of how the CPU switches between different tasks. Similarly, the thread abstraction hides the intricacies of managing multiple execution paths within a single process, allowing developers to focus on the logic of concurrent tasks.
Adding new capabilities:
- Example: Processes provide isolation between different applications, ensuring that one misbehaving application doesn’t affect others. Threads within a process allow for parallel execution of tasks, improving performance and responsiveness. Additionally, operating systems often provide inter-process communication (IPC) mechanisms, enabling processes to coordinate and share data.
Organizing information:
- Example: The process abstraction organizes the execution environment by encapsulating the code, data, and resources needed for a program to run. Threads further organize execution by dividing tasks within a process into smaller, manageable units. This organization helps in structuring complex applications and improving their maintainability.

Memory Address Space

Hiding undesirable properties:
- Example: The memory address space abstraction hides the physical memory layout from applications. Programs use virtual addresses, which the operating system maps to physical memory locations. This abstraction also hides the details of memory fragmentation and allocation, simplifying memory management for developers.
Adding new capabilities:
- Example: Virtual memory allows applications to use more memory than physically available by swapping data to and from disk. This capability enables larger and more complex applications to run on systems with limited physical memory. Additionally, memory protection mechanisms prevent one process from accessing the memory of another, enhancing system stability and security.
Organizing information:
- Example: The memory address space abstraction organizes memory into segments such as code, data, heap, and stack. This organization helps in managing different types of data efficiently and ensures that memory is used in a structured manner. For instance, the stack is used for function calls and local variables, while the heap is used for dynamic memory allocation.

Abstraction pros / cons

Advantages of OS providing abstractions?
- Allow applications to reuse common facilities
- Make different devices look the same
- Provide higher-level or more useful functionality
Challenges?
- What are the correct abstractions?
- How much should be exposed?

OS design requirements - what do we need?

Convenience, abstraction of hardware resources for user programs
Efficiency of usage of CPU, memory, etc.
Isolation between multiple processes
Reliability, the OS must not fail
Other:
- Security
- Mobility

What isn’t clear?

Comments? Thoughts?